1. Field of the Invention
The present invention relates to a control system for controlling braking forces applied to vehicle wheels in an automotive vehicle, which system is applied to electronically controlled brakes, and particularly to an automotive brake control system employing an external brake fluid pressure source for optimally controlling braking force applied to vehicle wheels, in which a traction control is performed during quick starting, for example, and an anti-skid brake control is performed during braking on a low frictional road surface. Specifically, the present invention relates to an automotive brake control system employing a brake fluid pressure control actuator applied commonly to both a traction control system and an anti-skid brake control system.
2. Description of the Prior Disclosure
Recently, there have been disclosed and developed various automotive brake control systems, namely a traction control system generally abbreviated as a "TCS" which is provided for suppressing excessive driving force exerted on driven wheels during quick depressing operation of the accelerator pedal, such as quick starting, quick acceleration, or the like, an anti-skid brake control system generally abbreviated as an "ABS" which is provided for preventing brakes from locking vehicle wheels during quick braking or during braking on a low frictional road surface so as to provide maximum effective braking, and a servo-assisted brake control device which is provided for generating a wheel-cylinder pressure higher than a master-cylinder pressure at a relatively light brake pedal pressure. One such conventional automotive brake control system has been disclosed it. Japanese Patent First Publication Tokkai (Showa) 58-122246.
The previously noted conventional brake control system includes a first pump unit serving as an external brake fluid pressure source, a brake fluid pressure control valve, and a second pump unit for returning brake fluid temporarily stored in a brake fluid reservoir to an accumulator. In addition to the above, the conventional brake control system includes a brake fluid pressure control actuator provided for adjusting a wheel-cylinder pressure on the basis of the fluid pressure generated by the external brake fluid pressure source, i.e., the first pump unit, during the traction control operation, and for adjusting the wheel-cylinder pressure on the basis of the master-cylinder pressure during the anti-skid brake control operation. In such a conventional brake control system, when the wheel-cylinder pressure must be reduced during the traction control or the anti-skid brake control, the above fluid pressure control valve is shifted to a pressure reducing position wherein the brake fluid in the wheel-cylinder is returned to the brake fluid reservoir. Simultaneously, the brake fluid stored in the brake fluid reservoir is fed to the accumulator by means of the second pump unit, so as to stably supply a sufficient amount of brake fluid with a desirable fluid pressure to the wheel-cylinder, during pressure-increasing operation in wheel-cylinders, such as quick braking. However, the aforementioned conventional brake control system is designed to operate the second pump unit when the wheel-cylinder pressure is reduced during the traction control operation or the anti-skid brake control operation.
As is generally known, the control cycle of the TCS is determined depending on the operating time of quick acceleration or quick starting. On the other hand, the control cycle of the ABS is determined depending upon the operating time of quick braking or braking on a low frictional road surface. Additionally, the increase in the wheel-cylinder pressure and the decrease in the wheel-cylinder pressure are alternately repeated at a relatively short cycle, during the anti-skid brake control. As appreciated from the above, the control cycle of the TCS is generally longer than that of the ABS. Therefore, when the wheel-cylinder pressure is reduced during a traction control, the pressure reducing time becomes longer rather than during an anti-skid brake control. During such a traction control with a wheel-cylinder pressure reduction, the vehicle occupants may feel uncomfortable, due to undesirable noise and vibration created by rotation of a drive motor having a driving connection with the second pump.
To avoid such discomfort, it is possible to arrange it so that the second pump is in an inactive state during the traction control with the wheel-cylinder pressure reduction, and that the second pump is in an active state only during the anti-skid brake control with the wheel-cylinder pressure reduction. In this case, supposing that the traction control operation is continuously performed, the great amount of brake fluid is stored in the reservoir just after the traction control ends, since there is no brake fluid supply from the reservoir to the accumulator in the inactive state of the second pump. Under this condition, supposing that the anti-skid brake control is performed, it is difficult that the brake fluid in the wheel-cylinder returns the reservoir when the wheel-cylinder pressure is reduced in accordance with the anti-skid brake control. That is, the wheel-cylinder pressure cannot be smoothly reduced, thereby causing a relatively long time for a brake fluid pressure reduction when the wheel-cylinder pressure reduction is required during the anti-skid brake control. This results in an undesirable low step-response during the anti-skid brake control performed after consecutively performed traction control operations.